The manufacture of phosphoric acid by the wet process which involves reacting phosphate rock of 30-39% P.sub.2 O.sub.5 content with concentrated sulfuric acid to release phosphoric acid and the recovery of the phosphoric acid therefrom, with the conversion of the sulfuric acid to insoluble calcium sulfate, requires in one way or another the balancing of a number of variables, such as free sulfuric acid, temperatures and principally the recovery of phosphoric acid of a given level of concentration and the disposal of the calcium sulfate either as gypsum or as hemihydrate. Generally, plants will be designed to produce the gypsum by-product or the hemihydrate by-product and with this variable the concentration of phosphoric acid which can be produced is essentially fixed. The characteristic gypsum process plant will produce a 32% P.sub.2 O.sub.5 acid, whereas the hemihydrate plant characteristically will produce a 45-50% P.sub.2 O.sub.5 product. Generally, operation of the plant in either mode can be judged for its efficiency by the steadiness by which it will produce an acid of the mean concentration, which, of course, is related to the phosphate rock used. Generally, a phosphate rock of about Kola grade namely 30-39% P.sub.2 O.sub.5 is preferred.
The various processes for producing wet process phosphoric acid are well known and have been discussed in a great deal of detail in various publications, the best known being the Fertilizer Science and Technology series Volume 1, Part 1, "Phosphoric Acid,"edited by A. V. Slack and published by Marcel Dekker.
The commonly used process is referred to as the gypsum or dihydrate process. This process produces a product acid which contains 30-32% P.sub.2 O.sub.5 in solution and 1.5-3.0% sulfuric acid plus assorted soluble impurities including iron, aluminum and fluosilicic acid in the product and in some cases a lower percentage of other impurities.
The disadvantages of higher corrosion rates, greater fume problems, difficulty with filtration and higher losses of P.sub.2 O.sub.5 in the calcium sulfate waste have generally deterred operators from using the hemihydrate process.
These disadvantages are now being overcome by improvements in materials of construction, operating technology and improved equipment. Perhaps the principle improvement in equipment is the development of the Landskrona continuous travelling belt filter.
It is a basic object of this invention to provide a means for expanding or more than doubling the capacity of an existing phosphoric acid plant by essentially duplicating the facility and interconnecting the new with the old to provide a means for operation in either the gypsum of the hemihydrate mode to produce a higher than 50% P.sub.2 O.sub.5 acid.
It is another object of the invention to provide a means for the operation of the phosphoric acid plant to enhance the recovery of P.sub.2 O.sub.5 from the phosphate rock, when operating to produce concentrated (45% P.sub.2 O.sub.5) phosphoric acid.
Other objects and advantages of the invention will in part be obvious and in part appear hereinafter.